Re: Tuning the fractional rig on a P19

Ted Duke (tduke@rockbridge.net)
Wed, 24 Nov 1999 21:12:42 -0500


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West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
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Judy and gang,

I don't even have time to digest this whole message, BUT, Never
say NEVER! Never say ALL! My short mast Potter 19 has only one
side stay per side, and NO spreader. Will read the message
again later and expound or argue with any of you. <bg>

House full of children and grandchildren.

Later,

Ted Duke
WWPs19 #626 The Duchess
Mountains of Virginia

JBlumhorst@aol.com wrote:
>
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> West Wight Potter Website at URL
> http://www.lesbois.com/wwpotter/
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> Hi webgang,
>
> It's been too quiet around here lately. Keel trunk is being fixed, can't go
> sailing. Hmmm, what to do?
>
> I'm probably gonna regret opening this can of worms and find my mailbox
> stuffed full of questions and disagreements about this, but take a look at
> two posts I wrote about tuning the P19 rig on the Trailor Sailor Board.
>
> The text follows:
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Post 1.
>
> There are indeed two side shrouds on all Potter 19's. The upper attaches to
> the same place as the fractional forestay and the lower attaches
> approximately even with the draft of the mainsail. Both attach to a chain
> plate a little aft of the maststep.
>
> According to my rigger, the Potter 19 is rigged in a classic fractional
> manner. The upper shroud provides primary lateral support of the mast column,
> as well as aft tension against headstay loads (in conjunction with the
> backstay). According to him, the primary purpose of the lower shroud is to
> allow you the option of tuning the rig for static and/or dynamic mast bend.
> The lower shrouds don't need to be very tight in order to provide primary
> lateral support of the mast column. That's the job of the upper shrouds.
>
> So why are they there? In combination with the headstay and backstay, the two
> aft-anchored side shrouds can be used to induce mast bend. If you bend the
> mast forward at the middle, you flatten the draft and can depower the sail
> far more effectively than you can by just tightening the outhaul.
>
> The upper shrouds on my boat are tuned at about 15-18% of breaking strength.
> The lowers are tuned between 10 and 12% of breaking strength. If you hit the
> upper shrouds sharply with your hand or a wrench, it makes a low-pitch
> musical tone. If you hit the lower shroud, it is too loose to make a musical
> note, but not loose enough to be floppy ( A floppy shroud makes floppy makes
> a "flup-flup-flup" sound). With enough tension on the forestay, this produces
> about 1" of mast pre-bend forward at the middle-lower section of the mast
> where the draft of the mainsail is deepest.
>
> When I tighten the backstay (my backstay is adjustable) for pointing or heavy
> winds, the middle of the mast moves forward another inch. This tightens the
> lower shroud, limiting the amount of mast bend I can induce, and (now that
> it's under tension) stabilizing the mast laterally in the mid-column section
> during heavy wind. It also flattens the draft in the main about 3 inches,
> which significantly reduces heeling and improves performance in high winds or
> pointing (same thing, it's now high "apparent wind"). The boat sails flatter
> in heavy wind, and points higher in medium to heavy wind too.
>
> So the two shrouds are a positive advantage, giving you some tuning options
> that you wouldn't have otherwise.
>
> You don't have to tune the shrouds on a Potter the way I do, if you don't
> want to fuss with bending the mast. You can tension the lowers just a little
> less than the uppers (producing a slightly lower pitched musical note than
> the uppers do) and still get plenty of support for the mast column in heavy
> winds. (Note for P19-ers - If you have concerns about the strength of the
> masthead fitting, you still get alot of performance improvement by tuning
> this way, even if you don't have an adjustable backstay)
>
> (BTW, those plastic shroud covers make it a harder to produce the "musical
> note" when you hit them. I cut my plastic covers off short so they cover the
> shroud only where it rubs against the gelcoat during trailering. It's better
> for the shrouds too, since stainless steel needs exposure to oxygen or else
> it corrodes quickly.)
>
> And just to clarify things -- In addition to the upper and lower side
> shrouds, most P19's have two dinky "babystays" for mastraising and lowering,
> running from somewhere on the lower section of the mast to the cabin top,
> directly outboard of the maststep pivot point. All these are designed to do
> is keep the mast from swinging sideways during raising and lowering.
>
> Most folks leave all the shrouds attached for trailering except for the
> forestay. Only minimal extra work is involved in coiling up the extra shroud
> for trailering once the mast is down.
>
> >From my perspective, the two shrouds give the boat a heck of a lot more heavy
> air versitility in return for the minimal extra work of coiling them up and
> bungee-ing them to the mast for trailering.
>
> Fair winds, Judy B
>
> ~~~~~~~~~~~~~~~~~~~~
> Post #2
>
> The Potter 19 has one upper and one lower. There is one set of shrouds
> attached to the mast at the level of the forestay, and one about half way
> down from there. They both attach at the bottom to the same chain plate
> slightly aft of the mast, at about a 20-30(?) degree mast-to-shroud angle.
> The upper shroud stabilizes the mast columm laterally, while the function of
> the lowr shroud is to limit forward mast bend and to stabilize the middle of
> the column in high winds.
>
> The rest of the rig: Fractional rig. No spreaders. Beefy mast compared to the
> righting moment of the hull (which is what counts when you're engineering the
> standing rigging. Backstay to masthead (on all but the stripped down "small
> lake" version of the boat for $8000 new). Very efficient type of rig
> according to Chapter 18 of Bethwaite's _High Performance Sailing_, because
> the rig adapts dynamically to increasing wind by (1) bending the masthead aft
> and allowing the top to twist off, (2) tightening the forestay and flattening
> the headsail, and (3) bending the middle of the mast forward to flatten the
> draft of the mainsail.
>
> A rig like this doesn't require as many frequent adjustments in changing or
> gusty winds. It dynamically responds with the correct de-powering response
> the "fast sense", keeping the boat balanced and sailing flat and fast.
>
> See my other post in this thread for tips on tuning a rig like this. It has
> to be tuned correctly to permit this type of dynamic response.
>
> Fair winds, Judy B